There is conventionally known a type of injection molding machine that obtains a plastic molding product by driving a screw with a motor, filling a melted resin by operations of the screw in a mold, and keeping a pressure of the resin. This conventional injection molding machine is equipped with a pressure sensor such as a load cell on the screw, and performs a pressure control on the resin such that the pressure of the resin follows a pressure command value that is externally given based on a control loop using a pressure detection value output from the pressure sensor.
In the pressure control described above, it is necessary to set a control parameter such as a gain to an appropriate value. Particularly in a pressure keeping operation, if the control parameter is larger than the appropriate value, it causes a problem that a pressure response (a time response waveform) becomes overshot with respect to a target pressure command or the pressure response exhibits an oscillating behavior.
When the pressure response shows an overshoot or an oscillation in the pressure keeping operation, a pressure larger than an intended pressure is applied. As a result, the dimension of the molded product may become too large or a flash or a mold release failure may occur. Furthermore, in the worst case, an oscillation occurs in a continuous manner in the pressure response, and this can cause a control system to be unstable.
In order to prevent the overshoot or the oscillation from occurring in the pressure response and to ensure a following characteristic to the pressure command value to a certain extent, it is necessary to manually adjust the control parameter for the pressure control by trial and error while checking the pressure response, which causes a problem that the time required for the adjustment increases.
In addition, there is another problem that an appropriate control parameter depends on the shape of the mold or the type of the resin, so that the appropriate control parameter needs to be adjusted for each shape of the mold and each type of the resin.
To deal with these problems, a control device for an injection molding machine is disclosed in Patent Literature 1, which differentiates an operational pattern such as a pressure command value and determines a control parameter for a pressure control in a continuous manner with zero of the obtained differential value as a basis.